1. Field of the Invention
The present invention relates to a channel estimation method and a channel estimation apparatus in a wireless communication system using a multicarrier, and more particularly to a channel estimation method and a channel estimation apparatus in an Orthogonal Frequency Division Multiplexing (OFDM) system, and a receiver using the same.
2. Description of the Related Art
Nowadays, with the development of communication industry and an increasing user demand for a packet data service, a need for a communication system capable of efficiently providing a high-speed packet data service is increasing. Since the existing communication networks have been developed mainly for the purpose of a voice service, they have a drawback in that their data transmission bandwidths are relatively small and their usage fees are expensive. In order to solve this drawback, research has been rapidly undertaken for use of an OFDM scheme which is a wireless access scheme to provide expanded bandwidth.
The OFDM scheme is a multicarrier transmission scheme in which a serial input symbol stream is converted into parallel signals. Converted parallel signals are modulated with multiple orthogonal subcarriers and the modulated signals are transmitted. The OFDM scheme has widely been exploited for digital transmission technologies requiring high-speed transmission, such as broadband wireless Internet technology, Digital Audio Broadcasting (DAB) technology, Wireless Local Area Network (WLAN) technology and so forth.
Estimation of a channel over which a radio signal is transmitted in an OFDM system includes a pilot signal-based estimation method, a method using data decoded in a decision directed scheme, and a method using a blind detection scheme for estimating a channel without known data. In general, supposing coherent demodulation is used in a wireless communication system, a transmitting end transmits pilot signals for channel estimation, and a receiving end for performing the coherent demodulation estimates a channel based on the received pilot signals.
Reference will now be made to how to arrange pilot signals in a transmission frame of a conventional OFDM system, with reference to FIGS. 1A to 1C. In a conventional OFDM system, a pilot arrangement scheme may be classified into a block-type pilot arrangement scheme, a comb-type pilot arrangement scheme and a lattice-type pilot arrangement scheme, according to whether pilot signals are arranged along the frequency axis, the time axis or both. FIG. 1A illustrates the block-type pilot arrangement scheme, FIG. 1B illustrates the comb-type pilot arrangement scheme, and FIG. 1C illustrates the lattice-type pilot arrangement scheme.
In FIGS. 1A to 1C, the abscissa represents time, the ordinate represents frequency, and each shaded portion P1 represents a pilot signal.
In the block-type pilot arrangement scheme illustrated in FIG. 1A, pilot signals are arranged at specific OFDM symbols along the time axis, respectively, and are arranged at all subcarriers of the OFDM symbols when viewed along the frequency axis. This scheme requires conducting interpolation in the time axis in order to estimate channels affecting data signals. In the comb-type pilot arrangement scheme illustrated in FIG. 1B, pilot signals are uniformly distributed over respective OFDM symbols, and are arranged at the same subcarrier at each time interval. This scheme requires conducting interpolation in the frequency axis in order to estimate channels affecting data signals. In the lattice-type pilot arrangement scheme illustrated in FIG. 1C, pilot signals are regularly arranged along both the time and frequency axes. This scheme requires conducting interpolation in both the time and frequency axes in order to estimate channels that are suitable for a variable channel environment and affect data signals.
Hereinafter, a description is provided of how to perform channel estimation for e.g. a DVB-H frame when pilot signals are arranged according to fixed rules on both the time and frequency axes. In the DVB-H frame illustrated in FIG. 2, pilot signals are arranged using a combination of the comb-type pilot arrangement scheme and the lattice-type pilot arrangement scheme. This combination scheme requires conducting interpolation in both the time and frequency axes in order to estimate a channel that is suitable for a variable channel environment and affects data signals.
With regard to this, when a pilot spacing in the time axis is compared with that in the frequency axis, a conventional interpolation technique begins with in the axis where a pilot spacing is narrower. That is, interpolation is conducted first in the axis where a pilot spacing is narrower, and then is conducted in the axis where a pilot spacing is wider. Since known channel information occurs when interpolation is conducted in the axis where a pilot spacing is wider, an interpolation interval is reduced as compared with the pilot spacing. In other words, when interpolation is conducted first in the axis where a pilot spacing is narrower, channel information for some data portions is acquired through the interpolation. This channel information may correspond to the same data positions when viewed in the axis where a pilot spacing is wider, and thus pilot portions and some data portions become known at the moment when interpolation is conducted in the axis where a pilot spacing is wider. Thus, an actual interpolation interval is reduced as compared with a pilot spacing.
FIG. 3 illustrates the sequence of interpolation operations for channel estimation in a conventional OFDM system. If it is assumed that the OFDM system is a DVB-H system having the frame structure illustrated in FIG. 2, a pilot spacing in the time axis is 4 symbols, as indicated by reference numeral “301”, and a pilot spacing in the frequency axis is 12 symbols, as indicated by reference numeral “303”. Accordingly, the interpolation designated by arrow number {circle around (1)} is conducted first in the time axis, and then interpolation designated by arrow number {circle around (2)} is conducted in the frequency axis. Thus, in order to conduct interpolations {circle around (1)} and {circle around (2)}, at least pilot information for interpolation in the time axis must be provided. Further, since interpolation in the frequency axis is conducted using result values from interpolation in the time axis, the result values must also be stored in a memory. Thus, in order to conduct the interpolation described in regard to FIG. 3, a memory capacity of, for example, (5×the number of pilot positions×data format) is required. Here, the pilot position includes not only the pilot position of one OFDM symbol, but all the positions of scattered pilots which are cyclically repeated.
More specially, in the above-mentioned memory capacity of five times the number of pilot positions multiplied by the data formats, numeral “5” denotes channel information for 2 pilot portions, which are consecutive when pilot signals are repeated every 4 symbols in the time axis, and channels of 3 data portions, which are acquired by interpolation using the channel information for 2 pilot portions. The number of pilot positions denotes the number of pilot positions including all the pilot positions of four OFDM symbols corresponding to a four cycle repetition, and the data format denotes the number of bits required for representing one channel information.
In the interpolation technique described above, since data of OFDM symbols corresponding to a pilot spacing must be stored, a high memory capacity is required for conducting interpolation in the axis where a pilot spacing is narrower. That is, in FIG. 3, a pilot spacing of 4 symbols in the time axis is relatively small, but a memory capacity capable of storing at least 5 symbols is required.
Further, the conventional interpolation technique is limited in regard to ensuring performance in a wireless environment where a terminal moves at high speed. That is, the performance of the terminal deteriorates because an interpolation interval in the time axis is fixed, despite the increased change of fading in the time axis as the speed at which the receiver moves increases. Further, for example, the frame structure in FIG. 2 requires pilot information for a spacing of at least 4 OFDM symbols in order to conduct interpolation in the time axis, and must use pilot information for at least 8 OFDM symbols in order to create known channel values at regular positions in the frequency axis. Thus, the number of OFDM symbols affecting interpolation in the frequency axis is 8 or more, which is not narrow in comparison with a coherent time as the transmission speed goes higher and higher.
Accordingly, the channel estimation method in a conventional OFDM system has a problem in that the performance of a receiver deteriorates because an interpolation interval in the time axis is fixed, and a high memory capacity is required for conducting interpolation in the frequency axis. Consequently, there is a need for a solution to this problem.